It has been found desirable to convert the information in a film, such as an x-ray film, to an electrical signal to allow the information to be stored in a computer. For this purpose, systems have been developed which read the information in a film and convert the information to digital signals for storing in a computer. A problem in digitizing film is that the film has regions with high optical density (low transmittance) and regions with low optical density (high transmittance). The regions with high optical density must be measured with great accuracy. This puts extreme requirements on the dynamic range of the system being used to do the digitizing. Dynamic range is defined as the ratio of the largest transmittance signal which can be handled by the system to the noise in transmittance near zero. The dynamic range is limited by the number of electrons which can be held in each detector pixel, by the noise in the detector read out means, and by the resolution of the analog-to-digital (A/D) converter utilized in the system to convert the signal from the detector to digital form. For example, in the scanning of radiographic film, the density range in this type of film is roughly 3.5 (a transmittance range of 1.0 to 0.0003). Also, it is generally felt that a signal-to-noise ratio greater than 15:1 is required at the maximum density. This translates to a dynamic range requirement of 50,000. Such a dynamic range is not possible with most standard detectors.
One approach which has been suggested to provide a large dynamic range is described in the article of R.M. Rangayyen et al. entitled "Expanding the dynamic range of x-ray vidodensitometry using ordinary image digitizing devices", published in APPLIED OPTICS, Vol. 23, No. 18, Sep. 15, 11964, pages 3117-3120. The article teaches reading the image twice, once at high density, and a second time at a lower density. The digital information from the two readings is combined to calculate a resulting image with a wide dynamic range. This article describes carrying out the method using a TV camera tube as the detector. Also, the device is operated in the usual area detector mode, with the film and detector being stationary. This technique suffers from the poor resolution and noise performance of the TV camera tube, and the tendency of the tube to bloom around regions of high transmittance.
Another approach is described in U.S. Pat. No. 5,034,825 (Ejiri et al.), issued Jul. 23, 1991, entitled "High Quality Image Sensor". The method described in this patent, like the method described in the Rangayyan et al. article, reads the image twice, one at a high density and a second time at a low density. The method of this patent has the advantage that it uses a solid state charge-coupled device (CCD) scanner as the detector. However, the method of this patent requires the film to be passed across the detector twice. This slows down the overall process, and there can be registration problems in moving the film across the detector a multiple of times.
It is desirable to be able to relatively rapidly read information on a film with regions of different optical densities and convert the information to an electrical signal.